1. Technical Field
The present invention relates to an improved quadrupole mass spectrometer array for the separation of ions with different masses.
2. Background Art
The quadrupole mass spectrometer ("QMS") was first proposed by W. Paul (1958). In general, the QMS separates ions with different masses by applying a direct current ("dc") voltage and a radio frequency ("rf") voltage on four rods having hyperbolic or circular cross sections and an axis equidistant from each rod. Opposite rods have identical potentials. The electric potential in the quadrupole is a quadratic function of the coordinates.
Ions are introduced in a longitudinal direction through a circular entrance aperture at the ends of the rods and centered on a midpoint between rods. Ions are deflected by the field depending on the ratio of the ion mass to the charge of the ion ("mass/charge ratio") and, by selecting the applied voltage and the amplitude and frequency of the rf signal, only ions of a selected mass/charge ratio exit the QMS along the axis of a quadrupole at the opposite end and are detected. Ions having other mass/charge ratios either impact the rods and are neutralized or deflected away from the axis of the quadrupole. As explained in Boumsellek, et al. (1993), a solution of Mathieu's differential equations of motion in the case of round rods provides that to select ions with a mass m, using an rf signal of frequency f and rods separated by a distance R.sub.o, the peak rf voltage V.sub.o and dc voltage U.sub.o should be as follows: EQU V.sub.o =7.219 m f.sup.2 R.sub.o.sup.2 EQU U.sub.o =1.212 m f.sup.2 R.sub.o.sup.2
Conventional QMSs weigh several kilograms, have volumes of the order of 10.sup.2 cm.sup.3, and require 10-100 watts of power. Further, vacua in the range of 10.sup.-6 -10.sup.10 torr are needed for satisfactory signal-to-noise ratio, due to the large free mean path required to transverse the pole length. Commercial QMSs of this design have been used for characterizing trace components in the atmosphere (environmental monitoring), in automobile exhausts, thin film manufacture, plasma processing, and explosives/controlled-substances detection. Such conventional QMSs are not suitable, however, for spacecraft life support-support systems and certain national defense missions where they have the disadvantages of relatively large mass, volume, and power requirements.
To meet these needs, a miniature QMS was developed by Ferran Scientific, Inc. (San Diego, Calif.). The Ferran QMS uses a miniature array of sixteen rods comprising nine individual quadrupoles. The rods are supported only at the detector end of the QMS by means of powdered glass that is heated and cooled to form a solid support structure. The dc and rf electric potentials are applied by the use of springs contacting the rods. The Ferran QMS dimensions are approximately 2 cm diameter by 5 cm long, including a gas ionizer and detector, with an estimated mass of 100 grams. The reduced size of the Ferran QMS results in several advantages, including a reduced power consumption of approximately 10 watts and the ability to operate at a higher operating pressure of approximately 1 mTorr.
The Ferran QMS was analyzed by Boumsellek, et al. (1993) and it was determined that its resolution was approximately 2.5 amu in the mass range 1-95 amu. This is a relatively low resolution for a QMS, making the miniature Ferran QMS only useful for commercial processing (e.g. chemical-vapor deposition, blood-plasma monitoring), but not for applications that require accurate mass separation, such as spacecraft life-support systems. The low resolution was traced to the fact that the rods were aligned only to within a 2% accuracy, whereas an alignment accuracy in the range of 0.1% is necessary for a high resolution QMS (Boumsellek et al. 1993). In addition, the ratio of rod radius to one-half the distance between rods having the same polarity (the "kissing circle" radius) of the Ferran QMS was measured to be about 1.46, whereas the ideal ratio is 1.16 (Boumsellek et al. 1993). It is these and other disadvantages of the Ferran QMS that the present invention overcomes.